The invention relates to improvements in apparatus for spray coating substantially plate-like or panel-shaped workpieces, and more particularly to improvements in apparatus for electrostatic spray coating edge-supported substrates which can constitute circuit boards and like components of printed circuits.
It is known to mount plate-like or panel-shaped substrates on a conveyor which is caused to transport the substrates through one or more spray coating zones. As a rule, the conveyor is provided with clamps or like engaging elements which further serve to connect the substrates to the ground. It is important to prevent charging of the substrates by charge carriers which are caused to impinge upon the substrates in the course of an electrostatic spray coating operation, irrespective of whether the material (such as lacquer or paint) which is to form a coat on the substrates is atomized by mechanical or pneumatic means. As a rule, the substrates are grounded by way of the conveyor which transports them through one or more spray coating zones. Reference may be had to German Pat. No. 24 46 767 to Brennecke et al. which discloses an overhead conveyor with hooks on which the substrates are suspended and which transport the substrates through the spray coating zone. The patent further points out that the critical factor in ensuring adequate grounding of conveyed substrates is the manner of attaching the substrates to the conveyor. The reason is that, in the absence of any undertakings to the contrary, the suspending elements of the conveyor become coated with atomized material and their contact or transition resistance increases; this causes undesirable charging of conveyed workpieces. The patentees propose to solve the problem by causing the workpieces to advance past a contact-free discharge unit, at least during movement through the spray coating station.
U.S. Pat. No. 4,643,129 to Sari discloses a system for the treatment of edge-supported substrates, such as printed electronic circuit boards. The patentee does not discuss the problem of adequately grounding the workpieces but is concerned exclusively with the provision of a conveyor which operates without lubrication. The reason is that the patentee proposes to subject substrates to a treatment (such as baking, solvent degreasing, spray coating, drying and the like) which has a deleterious effect upon the lubricant.
German Auslegeschrift No. 25 17 504 of Buschor discloses an electrostatic spray coating apparatus for bottles and like hollow workpieces. The inventor proposes to introduce the grounded electrode into the interior of the hollow workpiece close to the bottom wall and to seal the neck of the workpiece so as to prevent penetration of atomized coating material into the interior of the workpiece.
A similar proposal, is disclosed in German Offenlegungsschrift No. .times.01 824 of O'Connel who describes a conveyor which advances parts of automotive vehicles through an electrostatic spray coating station. The supports for the workpieces extend in part into the workpieces and have embedded therein conductor plates or contain conductive pulverulent material so as to establish electrical connections with frames at the undersides of the supports. The frames are grounded through the medium of carriers which extend downwardly and are connected to a chain conveyor or the like.
U.S. Pat. No. 2,684,656 to Ransburg proposes to ground the articles to be spray coated through the medium of the conveyor and to tilt the articles during transport through the electrostatic spray coating station.
German Auslegeschrift No. 2 204 982 of Suhrkamp et al. discloses a method of spray coating articles of glass or ceramic material while the articles are still hot and are grounded by way of their conveyor. The manner of preventing the atomized material from interfering with adequate grounding of the articles is not discussed.
German Offenlegungsschrift No. 34 20 453 of Hell discloses an apparatus for applying different coats to opposite sides of window and door frames. The problem of properly grounding the workpieces during treatment is not discussed by the inventor. He proposes to place partitions between a series of workpieces so as to prevent penetration of one coating material from one side to the other side of the conveyor and vice versa.
German Offenlegungsschrift No. 33 38 328 of Maldonado et al. discloses a continuous automatic film developing apparatus wherein the leaders of successive dental x-ray films are deflected by concave surfaces so as to avoid the need for engagement of the edges of successive films in a manner as shown in FIGS. 3 and 4 of this printed publication. The problem of ensuring adequate grounding does not arise in the apparatus of Maldonado et al.
Commonly owned copending United States patent application Ser. No. 095,319 filed Sept. 10, 1987 by Heine et al. discloses an electrostatic spray coating apparatus wherein an endless apertured conveyor advances the workpieces beneath a spray coating station and its upper reach is located above a slowly running web serving to intercept the coating material which penetrates through the openings of the conveyor.
The assignee of the present application is the owner of numerous pending United States patent applications which deal with certain components of electrostatic spray coating apparatus. Thus, the copending patent application Ser. No. 942,164 filed Dec. 16, 1986 by Kuhn et al. discloses a portable electrostatic spray gun, the copending patent application Ser. No. 940,623 filed Dec. 11, 1986 by Kuhn discloses an apparatus for simultaneously pumping a plurality of liquids, the copending application Ser. No. 159,115 filed Feb. 23, 1988 by Kuhn et al. discloses a nozzle assembly for spray guns, and the copending patent application Ser. No. 159,225 filed Feb. 23, 1988 by Schlinkheider et al. discloses a fluid flow machine. A spraying method and a device for applying a film to a workpiece are disclosed in commonly owned U.S. Pat. No. 4,713,257 granted Dec. 15, 1987 to Luttermoller.